Fix PICO_STDIO_USB_SUPPORT_CHARS_AVAILABLE_CALLBACK (#1216)

It's being used in the wrong place
diff --git a/src/rp2_common/pico_stdio_usb/stdio_usb.c b/src/rp2_common/pico_stdio_usb/stdio_usb.c
index 2c58235..fda2fda 100644
--- a/src/rp2_common/pico_stdio_usb/stdio_usb.c
+++ b/src/rp2_common/pico_stdio_usb/stdio_usb.c
@@ -24,6 +24,11 @@
 static uint8_t stdio_usb_core_num;
 #endif
 
+#if PICO_STDIO_USB_SUPPORT_CHARS_AVAILABLE_CALLBACK
+static void (*chars_available_callback)(void*);
+static void *chars_available_param;
+#endif
+
 // when tinyusb_device is explicitly linked we do no background tud processing
 #if !LIB_TINYUSB_DEVICE
 // if this crit_sec is initialized, we are not in periodic timer mode, and must make sure
@@ -37,10 +42,6 @@
 #else
 static uint8_t low_priority_irq_num;
 #endif
-#if PICO_STDIO_USB_SUPPORT_CHARS_AVAILABLE_CALLBACK
-static void (*chars_available_callback)(void*);
-static void *chars_available_param;
-#endif
 
 static int64_t timer_task(__unused alarm_id_t id, __unused void *user_data) {
     int64_t repeat_time;